1. Field of the Invention
The present invention relates to a method of producing a planographic printing plate support and particularly relates to a method of producing an aluminum support having excellent electrolytic roughness.
2. Background
An aluminum or aluminum alloy plate has conventionally been used as a support for an off-set printing plate. In using an aluminum plate, it is generally necessary that the aluminum plate have a moderate adhesive property to a photosensitive material and a moderate water retentivity.
Therefore, the aluminum plate must be toughened so that it can have a uniform and delicately grained surface. Since this toughening treatment influences the printing characteristics and the durability of the printing plate, the effect thereof is an important factor in the production of the plate material.
As a method of toughening an aluminum support for a printing plate, there is generally employed an AC electrolytic etching method in which an ordinary sinusoidal alternating current or a special alternating waveform current such as an alternating rectangular waveform current is applied. Roughening of the aluminum plate is performed utilizing a suitable electrode such as a graphite electrode as a counter electrode. The roughening is generally completed after a single treatment. However, the depth of each pit obtained by such a toughening treatment is relatively shallow so that the resulting aluminum support is not durable. Therefore, various methods have been proposed so that a suitable aluminum plate can be obtained as a printing plate support having a grained surface in which the depth of each of the pits is larger than the diameter of the pit and the pits are evenly distributed.
Included in these methods is a roughening method using a special electrolytic electric source waveform (Japanese Patent Unexamined Publication No. Sho. 53-67507), a method in which the ratio of the quantity of electricity at the positive electrodes to the quantity of electricity at the negative electrodes at the time of electrolytic toughening is controlled with use of an alternating current (Japanese Patent Unexamined Publication No. Sho. 54-65607), a method in which an electric source waveform is applied (Japanese Patent Unexamined Publication No. Sho. 55-25381) and a method in which a combination of the quantities of current conduction per unit area is controlled (Japanese Patent Unexamined Publication No. Sho. 56-29699). Further, methods which include mechanical toughening (e.g., Japanese Patent Unexamined Publication No. Sho. 55-142695) are known.
On the other hand, as a method of producing an aluminum support, there is a method comprising the steps of casting a slab (with the thickness ranging from 400 to 600 mm, the width ranging from 1000 to 2000 mm and the length ranging from 2000 to 6000 mm) by melting and holding an ingot of aluminum; applying a facing attachment to an impurity structure portion of a surface of the slab to thereby cut the impurity structure portion by 3-10 mm; equally heating the slab in a soaking pit at a temperature ranging from 480.degree. C. to 540.degree. C. for a period of 6 to 12 hours in order to remove stress inside the slab and make the structure of the slab uniform and then hot-rolling the slab at a temperature ranging from 480.degree. C. to 540.degree. C. After the slab is hot-rolled into a thickness ranging from 5 to 40 mm, the slab is cold-rolled into a predetermined thickness at room temperature. Then, for homogenizing of the structure and for flattening a plate annealing is performed. Thereafter, cold rolling is carried out to obtain a predetermined thickness, and finally flattening is performed. The aluminum support thus produced is used as a planographic printing plate support.
In the case of electrolytic toughening treatment, however, the treatment is apt to be affected by the aluminum support to be subjected to the treatment. In particular, in producing the aluminum support through the steps of melting/holding, casting, facing and thermal equalizing, there arise a variety of components of a metal alloy contained in the surface layer even in the case where, not only are heating and cooling carried out alternately, but facing (i.e., cutting the surface layer) is provided. Accordingly, this causes the lowering of the yield rate of the electrolytic toughening treatment.
As a method for improving the yield rate in the electrolytic toughening treatment, the inventor of the subject application has proposed a method of producing a planographic printing plate support, characterized by the steps of: forming a thin-plate coil by continuously casting from molten aluminum; applying cold rolling, heat treatment and flattening to the coil to thereby obtain an aluminum support; and then toughening the aluminum support (U.S. Pat. No. 5,078,857).
However, this method still has not significantly improved the yield rate or the aptitude to roughening. In addition, stripe irregularities occur in the toughening-treated surface so that the external appearance is poor.
Accordingly, it was found that the aluminum grain size in the surface of the aluminum plate after final cold rolling or heat treatment greatly affected the quality of the surface after surface roughening.
Accordingly, an object of the present invention is to provide a method of producing a planographic printing plate support in which not only the quality of the material of the aluminum support is improved to thereby improve the yield in the electrolytic roughening treatment but the ability of the planographic printing plate to be toughened is also improved.
Another object is to provide a method which produces a planographic printing plate having excellent surface quality and yield after the surface toughening has been completed.
Yet another object of the invention is to provide a method of producing a planographic printing plate support in which stripe irregularity can be prevented from occurring in the roughened surface to thereby make it possible to produce a planographic printing plate excellent both in the aptitude to roughening and in external appearance.